In France, and in other countries, the electrical power available to the railway network varies depending on region. In general, there are regions provided with 25 kV AC power at 50 Hz, or 15 kV AC at 16.67 Hz, and regions provided with DC power at a voltage that is either 3 kV or else 1.5 kV.
To enable trains to travel equally well over all such regions, it is necessary to provide multicurrent locomotives, i.e. locomotives capable of taking power equally well from AC and from DC.
Multicurrent locomotives started to become genuinely successful with the use of converters based on gate turnoff (GTO) thyristors, as described in the article "Convertisseurs a GTO pour transports ferroviaires" [GTO converters for rail transport] by G. Wiegner, published in the journal ZEV, July 1989. That article shows that GTO thyristors have made it possible to develop so-called 1-quadrant, 2-quadrant, or 4-quadrant converters suitable for converting power supply current whether AC or DC into chopped current that is applied directly to the input of inverters that control the motors of the locomotive.
Thus, as explained in that article, the ICE machine of West German railways uses 4-quadrant power converters to supply AC, in which the midpoints of the two pairs of GTO thyristors are interconnected by the secondary winding of a transformer whose primary winding is connected to the AC power supply network.
Similarly, that article mentions series 252 bicurrent locomotives of Spanish railways (RENFE) using three 4-quadrant converters to supply AC, in which two of the converters are used as voltage-lowering choppers when running under a DC power supply at 3 kV.
Unfortunately, in that type of locomotive, each bogey includes two motors powered by a set of two or three converters and one or two inverters. The converters and the inverters of a single bogey have points in common (filter capacitors, GTO control, etc.) such that any failure causes tractive effort to be lost from both motors in the same bogey (where the most common failure of a converter is short circuiting between its three outputs). For a locomotive fitted with four motors, a failure thus leads to 50% of the effort being lost. When the locomotive is heavily loaded, it thus becomes incapable of finishing off the service. This disturbs the timetable for the train concerned and also disturbs other trains.